While organic electroluminescent (EL) devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light emitting diodes, or OLEDs. Representative of earlier organic EL devices are Gurnee et al. U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, “Double Injection Electroluminescence in Anthracene”, RCA Review, 30, 322, (1969); and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. The organic layers in these devices, usually composed of a polycyclic aromatic hydrocarbon, were very thick (much greater than 1 μm). Consequently, operating voltages were very high, often >100 V.
More recent organic EL devices include an organic EL element consisting of extremely thin layers (e.g. <1.0 μm) between the anode and the cathode. Herein, the term “organic EL element” encompasses the layers between the anode and cathode. Reducing the thickness lowered the resistance of the organic layers and has enabled devices that operate at much lower voltage. In a basic two-layer EL device structure, described first in U.S. Pat. No. 4,356,429, one organic layer of the EL element adjacent to the anode is specifically chosen to transport holes, and therefore is referred to as the hole transporting layer, and the other organic layer is specifically chosen to transport electrons and is referred to as the electron transporting layer. Recombination of the injected holes and electrons within the organic EL element results in efficient electroluminescence.
There have also been proposed three-layer organic EL devices that contain an organic light emitting layer (LEL) between the hole transporting layer and electron transporting layer, such as that disclosed by C. Tang et al. (J. Applied Physics, Vol. 65, 3610 (1989)). The light emitting layer commonly consists of a host material doped with a guest material, otherwise known as a dopant. Still further, there has been proposed in U.S. Pat. No. 4,769,292 a four-layer EL element comprising a hole injecting layer (HIL), a hole transporting layer (HTL), a light emitting layer (LEL) and an electron transporting/injecting layer (ETL). These structures have resulted in improved device efficiency.
Many emitting materials that have been described as useful in an OLED device emit light from their excited singlet state by fluorescence. The excited singlet state can be created when excitons formed in an OLED device transfer their energy to the singlet excited state of the dopant. However, only 25% of the excitons created in an EL device are singlet excitons. The remaining excitons are triplet, which cannot readily transfer their energy to the dopant to produce the singlet excited state of a dopant. This results in a large loss in efficiency since 75% of the excitons are not used in the light emission process.
Triplet excitons can transfer their energy to a dopant if it has a triplet excited state that is low enough in energy. If the triplet state of the dopant is emissive it can produce light by phosphorescence. In many cases, singlet excitons can also transfer their energy to the lowest singlet excited state of the same dopant. The singlet excited state can often relax, by an intersystem crossing process, to the emissive triplet excited state. Thus, it is possible, by the proper choice of host and dopant, to collect energy from both the singlet and triplet excitons created in an OLED device and to produce a very efficient phosphorescent emission. The term electrophosphorescence is sometimes used to denote electroluminescence wherein the mechanism of luminescence is phosphorescence.
Another process by which excited states of a dopant can be created is a sequential process in which a hole is trapped by the dopant and subsequently recombines with an electron, or an electron is trapped and subsequently recombines with a hole, in either case producing an excited state of the dopant directly. Singlet and triplet states, and fluorescence, phosphorescence, and intersystem crossing are discussed in J. G. Calvert and J. N. Pitts, Jr., Photochemistry (Wiley, N.Y., 1966) and further discussed in publications by S. R. Forrest and coworkers such as M. A. Baldo, D. F. O'Brien, M. E. Thompson, and S. R. Forrest, Phys. Rev. B, 60, 14422 (1999) and M. A. Baldo, S. R. Forrest, Phys. Rev. B, 62, 10956 (2000).
Emission from triplet states is generally very weak for most organic compounds because the transition from the triplet excited state to the singlet ground state is spin-forbidden. However, it is possible for compounds with states possessing a strong spin-orbit coupling interaction to emit strongly from triplet excited states to the singlet ground state (phosphorescence). For example, fac-tris(2-phenyl-pyridinato-N,C2′-)Iridium(III) (Ir(ppy)3) emits green light (K. A. King, P. J. Spellane, and R. J. Watts, J. Am. Chem. Soc., 107, 1431 (1985); M. G. Colombo, T. C. Brunold, T. Reidener, H. U. Güdel, M. Fortsch, and H.-B. Bürgi, Inorg. Chem., 33, 545 (1994)). Organic electroluminescent devices having high efficiency have been demonstrated with Ir(ppy)3 as the phosphorescent material and 4,4′-N,N′-dicarbazole-biphenyl (CBP) as the host (M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, S. R. Forrest, Appl. Phys. Lett., 75, 4 (1999), T. Tsutsui, M.-J. Yang, M. Yahiro, K. Nakamura, T. Watanabe, T. Tsuji, Y. Fukuda, T. Wakimoto, S. Miyaguchi, Jpn. J. Appl. Phys., 38, L1502 (1999)). Additional disclosures of phosphorescent materials and organic electroluminescent devices employing these materials are found in U.S. Pat. No. 6,303,238 B1, WO 2000/57676, WO 2000/70655, WO 2001/41512 A1, WO 2002/02714 A2, WO 2003/040256 A2, and WO 2004/016711 A1.
Mixed hosts have been used to improve the efficiency, voltage and operational stability of phosphorescent OLED devices. H. Aziz et al. in U.S. Pat. No. 6,392,250 B1, US 2003/0104242 A1 and US 2003/0134146 A1 disclose organic electroluminescent devices having an emissive layer containing the phosphorescent 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porhine Platinum(II) (PtOEP) dopant and an about equal weight percent of both NPB and Alq as host materials. R. Kwong et al. in US 2002/0074935 A1 also disclose devices with an emissive layer containing the PtOEP or bis(benzothienyl-pyridinato-N ^C)Iridium(III)(acetylacetonate) as a dopant and equal proportions of NPB and Alq as host materials. In US 2004/0155238 a light emitting layer of the OLED device contains a wide band gap inert host matrix in combination with a charge carrying material and a phosphorescent emitter. The charge carrying compound can transport holes or electrons, and it is selected so that charge carrying material and phosphorescent emitter transport charges of opposite polarity. However, in this case, blue OLED devices employing these disclosed materials require use of substantial amounts of the phosphorescent emitters and still do not solve the high voltage problem.
M. Furugori et al. in US 2003/0141809 disclose phosphorescent devices where a host material is mixed with another hole- or electron transporting material in the light emitting layer. The document describes that devices utilizing plural host compounds show higher current and higher efficiencies at a given voltage; however, reported luminance data are quite moderate. Efficient single-layer-solution processed phosphorescent OLED devices based on fac-tris(2-phenylpyridine)Iridium cored dendrimer are described in T. Anthopoulos et al., Appl. Phys. Lett., 82, 4824 (2003). T. Igarashi et al. in WO 2004/062324 A1 disclose phosphorescent devices with the light emitting layer containing at least one electron transporting compound, at least one hole transporting compound and a phosphorescent dopant. Various materials were tested as co-hosts for the blue and green emitters, and high efficiency devices are reported. However, luminous and power efficiencies of the disclosed OLEDs can be improved much further.
High emission efficiency in phosphorescent OLED devices with a neat host is usually obtained by incorporating a hole blocking material between the light emitting layer and the cathode in order to limit the migration of holes and confine electron-hole recombination and the resulting excitons to the light emitting layer (for example, see U.S. Pat. No. 6,097,147).
In addition to a hole blocking layer, a phosphorescent OLED device employing a neat host and a phosphorescent material may include at least one hole transporting layer with suitable triplet energy levels, placed adjacent the light emitting layer on the anode side, to help confine the electron-hole recombination events to the light emitting layer. This feature can further improve the efficiency of the device. Examples of hole transporting materials whose energy levels make them suitable for use with many phosphorescent materials include 4,4′,4″-tris(N-3 -methylphenyl-N-phenylamino)triphenylamine(MTDATA; see JP2003092186A), bis[4-(N,N-diethylamino)-2-methylphenyl](4-methylphenyl)methane (MPMP; see WO02/02714 and WO03/040257), N,N-bis[2,5-dimethyl-4-[(3-methylphenyl)phenylamino]phenyl]-2,5-dimethyl-N′-(3-methylphenyl)-N′-phenyl-1,4-benzenediamine (see JP2004 139819 A and US 2004/018910 A1). However, use of these materials alone does not give the optimum performance possible in an electroluminescent device.
M. Thompson et al. in US 2004/0048101 disclose phosphorescent blue and white OLED devices comprising an electron blocking layer and the light emitting layer with a neat host and a phosphorescent emitter. By inserting an electron blocking layer between the hole transporting and light emitting layers electron leakage can be eliminated and, hence, luminous efficiency is increased. Fac-tris(1-phenylpyrazolato, N,C2′)Iridium (III) (Irppz) and Iridium(III)bis(1-phenylpyrazolato, N,C2′)(2,2,6,6-tetramethyl-3,5-heptanedionato-O,O) (ppz2Ir(dpm)) have been disclosed as suitable electron blocking materials.
Notwithstanding all these developments, there remains a need to further improve efficiency and reduce drive voltage of the devices, as well as to improve other features such as operational stability.